The present invention relates to methods for forming monocrystalline III-V arsenide-nitride semiconductor materials, especially those materials having a high bandgap (greater than 2.0 eV). More particularly it relates to direct bandgap light emitting materials, preferably those with cubic crystal lattices, which can be used to make devices such as semiconductor lasers that emit light with frequencies in a range that more than span the visible spectra.
Semiconductor lasers that emit light in the long wavelength visible (red) and near infrared spectra have been known for many years. To date, however, it has been difficult to produce such lasers that emit light in the short wavelength, blue region of the visible spectra. A wide variety of applications await such xe2x80x9cbluexe2x80x9d lasers, should they become commercially available.
Certain II-VI semiconductor compounds such as zinc selenide sulphide (ZnSeS) have been considered promising candidates for blue lasers. In an article entitled xe2x80x9cBlue-green laser diodesxe2x80x9d, Applied Physics Letters, v. 59, Sep. 9, 1991, M. A. Haase et al. describe achieving lasing action at a wavelength of 490 nm with a ZnSe based structure under pulsed current injection at 77 K. More recent advances in this material system using a remote plasma source for the introduction of radical nitrogen (N) atoms have resulted in the demonstration of pulsed laser operation at room temperature at a wavelength of 480 nm.
The ZnSeS based structures have several limitations, however, due to conduction band alignment and the activation energy of defects. The confinement energy of the cladding layers limits efficient operation of the ZnSeS system to a wavelength of 530 nm, as the electron confinement is too small for efficient operation of laser diodes at 480 nm or shorter wavelengths. The low activation energy of defects in the crystal lattice of such a material limits the processing and growth temperature of the structure to no more than a few hundred degrees Celsius.
The ZnSeS materials are further complicated by the problems associated with growth on a gallium arsenide (GaAs) substrate. The interface between the ZnSeS and GaAs results in a cross doping of the compounds. The Ga and As act as dopants in the ZnSeS and the Zn, Se and S act as dopants in the GaAs. As a result the interface between the two compounds becomes quite resistive. Laser diodes fabricated with this interface require operating voltages in excess of 20 V. The heat dissipated from this is also high, preventing continuous wave (CW) operation at room temperature.
An alternative to the II-VI compounds for blue light emission are the III-V semiconductor compounds, such as AlGaInN. The advantages of the III-V material systems are several. First, they include materials having large direct bandgaps ranging from 2 eV to 6 eV. Second, large energy differences between the valleys of the lowest direct bandgaps and those of the lowest indirect bandgaps exist. Third, electrons and holes can be satisfactorily confined in heterostructures, including quantum well structures, due to large confinement energies. Fourth, production of these materials is compatible with metal organic chemical vapor deposition (MOCVD) growth reactors. Fifth, the materials typically have low resistance to n-type and p-type doping. Finally, the III-V materials can avoid the cross doping and defect creation and propagation problems that plague the II-VI materials.
AlGaInN materials have so far been grown only on mismatched substrates such as Si, SiC, single crystal Al2O3 and MgO. Mismatched crystal lattices tend to have defects that absorb light, lowering the efficiency of light generation and reducing lasing potential. Defects also tend to propagate through crystals, lowering the useful life of the crystals.
Another difficulty is that AlGaInN type materials typically have a native wurtzite, or generally hexagonal, crystal lattice, which has few convenient cleavage planes to form mirrored facets for Fabry-Perot reflection and which cannot be easily grown to exactly layered depths for quantum wells or other cladding confined structures. For laser diodes, it is desirable that the heterostructure material have a generally cubic zinc-blende crystal lattice. While in AlGaInN materials the cubic structure is metastable at typical production temperatures, and thus not impossible to form in principle, to actually produce such a cubic crystal lattice material by epitaxial growth techniques appears to require a substrate that is not only lattice matched to the AlGaInN type material but also has a cubic lattice that induces the growth of a cubic substrate lattice of the AlGaInN type material layers. Substrates which have commonly been used for group III-V high bandgap semiconductors, such as basal plane {0001} sapphire (Al2O3) and xcex1-SiC, have the disadvantage of producing the less desirable wurtzite crystal lattice structure.
In U.S. Pat. No. 5,146,465, Kahn et al. describe growing layers of AlGaN with alternating concentrations of Al and Ga on an AlN buffer layer which, in turn, was grown on an Al2O3 substrate. Kahn et al. appear to overcome some of the difficulties inherent in the wurtzite lattice by polishing walls of the device and adding mirrors. However, cleaved facets, when available, are inherently better mirror surfaces than etched or polished surfaces because of their lower defect density. In U.S. Pat. Nos. 5,173,751 and 5,274,251, Ota et al. take advantage of the lattice constant of xcex1-ZnO, which falls between that of several III-V nitride materials, allowing mixing of those materials in proportions calculated to match the substrate lattice of ZnO, to form AlGaInN or AlGaNP layers. The resultant crystal lattice appears to be wurtzite in form.
In U.S. Pat. No. 4,862,471, Pankove describes the growth on a gallium phosphide (GaP) substrate of gallium nitride (GaN). Indium nitride (InN) or aluminum nitride (AlN) layers are similarly grown to form a quantum well light emitting device. Similarly, U.S. Pat. No. 5,076,860 to Ohba et al. teaches a compound semiconductor material of GaAlBNP with a zincblende (cubic) crystal lattice, grown on a GaP substrate. Ohba et al. describe several different materials, including a GaAlN semiconductor grown on a BP substrate and the growth of group III-V materials having ordered bonds but non-lattice matched crystals. U.S. Pat. No. 5,042,043 to Hatano et al. describes a semiconductor laser formed from alternately stacking BP and GaAlN layers to form GaxAlyB1xe2x88x92xxe2x88x92yNzP1xe2x88x92z material on a GaP substrate. In all of these cases, the use of GaP as a substrate results in a severe lattice mismatch with the nitride material layers (about 20% mismatch for GaN). In an article entitled xe2x80x9cHigh-Efficiency Aluminum Indium Gallium Phosphide Light-Emitting Diodes,xe2x80x9d Hewlett-Packard Journal, August 1993, pp. 6-14, R. M. Fletcher et al. describe gallium arsenide phosphide doped with nitrogen (GaAsP:N) and gallium phosphide doped with nitrogen (GaP:N).
In an article entitled xe2x80x9cLuminescence quenching and the formation of the GaP1xe2x88x92xNx alloy in GaP with increasing nitrogen contentxe2x80x9d, Applied Physics Letters, Vol. 60, No. 20, May 18, 1992, pp. 2540-2542, J. N. Baillargeon et al. teach N doping in GaP of up to 7.6% using molecular beam epitaxy (MBE), and note that increasing nitrogen content tends to shift emission spectra lower above a certain nitrogen concentration. Similarly, X. Liu et al. describe observing a red shift in emission spectra from GaP:N as nitrogen (N) concentration is increased in xe2x80x9cBand gap Bowing in GaP1xe2x88x92xNx alloysxe2x80x9d, Applied Physics Letters, Vol. 63, No. 2, Jul. 12, 1993, pp. 206-210. Difficulties are encountered when mixed compounds having significant amounts of both nitrogen and other group V elements are attempted. Miyoshi et al. describe a miscibility gap for growth of GaP1xe2x88x92xNx for xxe2x89xa70.04 in an article entitled xe2x80x9cMetalorganic vapor phase epitaxy of GaP1xe2x88x92xNx alloys on GaPxe2x80x9d, Applied Physics Letters, Vol. 63, No. 25, Dec. 20, 1993, pp. 3506-3508.
Of primary concern to reliable visible laser diode operation is the optimization of the substrate and growth buffer layers. To form a semiconductor material having desirable lacing properties, it is advantageous to grow such a material on a lattice matched substrate in order to avoid the promulgation of defects that absorb light. It is also desirable, whenever possible, that the substrate promote growth in the semiconductor material of a cubic crystal lattice that facilitates the relatively easy formation of cleaved reflective facets for defining resonant laser cavities.
The present invention involves formation of monocrystalline III-V compound semiconductor materials having at least nitrogen and arsenic at group V lattice sites of the crystal material. Other group V elements, such as phosphorus and antimony, can also be present at group V lattice sites in addition to the nitrogen and arsenic. The group III atomic species can be any combination of boron, aluminum, gallium and indium.
The exact composition of a particular material to be produced, that is, the relative concentration of each of the group III and group V elements in the III-V compound, is generally selected so as to substantially lattice match with the selected substrate""s growing surface. Some lattice strain due to lattice mismatch greater that 1% can be accommodated provided the mismatched layer is sufficiently thin to avoid formation of lattice defects. In addition, for light emitting devices, such as laser diodes and LEDs, the desired emission wavelength determines the required semiconductor bandgap for the material and hence plays a major role in the choice of material composition and of a suitable substrate. For emission wavelengths shorter than about 620 nm, a bandgap greater the 2 eV is required. The III-V compound arsenide-nitride materials of the present invention are characterized by a large bandgap bowing parameter which must be taken into account when matching up the desired semiconductor bandgap with a possible material composition. GaAsxN1xe2x88x92x material, where xxe2x89xa60.10, has a bandgap greater than 2.0 eV. Laser diodes further require that the selected material be a direct bandgap material, that is, one where the energy of the lowest direct bandgap is below that of the lowest indirect bandgap, while LEDs are capable of operating using either direct or indirect bandgap materials. The GaAsxN1xe2x88x92x noted above is a direct bandgap material. The addition of other group III and group V elements, such as boron, aluminum, indium, phosphorus and indium, to the basic GaAsN composition allows somewhat independent adjustment of the lattice constant and bandgap, allowing different materials with different emission wavelengths to be lattice matched to the same substrate. For LIDAR systems, laser diodes using strained InGaAs1xe2x88x92yNy active regions, where yxe2x89xa60.04, can produce light emissions in the 2.0 xcexcm to 2.5 xcexcm range. It is also possible to produce electronic devices, such as transistors, which are capable of high temperature operation, using the high bandgap semiconductor material of the present invention.
The monocrystalline material of the present invention if formed by epitaxial growth on a monocrystalline substrate. Depending on the choice of substrate material and the orientation of the substrate""s crystal lattice with respect to the substrate""s surface, that is, the choice of crystal lattice plane for the growing surface, the resulting III-V compound arsenide-nitride material layers may have either a cubic zincblende crystal structure or a hexagonal wurtzite crystal structure. InGaAlAs materials have a native zincblende structure, but with the appropriate choice of substrate can be induced to grow in the metastable wurtzite structure. InGaAsAlN materials, on the other hand, have a native wurtzite structure, but with the appropriate choice of substrate can be induced to grow in the metastable zincblende structure. Substrates can be selected from the group consisting of Al2O3 (sapphire) using either basal plane or r-plane growing surfaces, diamond, Si, Ge, SiC in both wurtzite (xcex1) and zincblende (xcex2) forms, InN, GaP, GaAsP, GaAs, InP, ZnO, ZnS, and ZnSe.
The arsenide-nitride materials of the present invention generally favor growth of cubic zincblende crystal structures when arsenic is the majority component in the group V lattice sites, up to about 4 or 5 percent nitrogen and favor growth of hexagonal wurtzite crystal structures when nitrogen is the majority component in the group V lattice sites, up to about 20 percent arsenic. When growth techniques, such as metalorganic chemical vapor deposition (MOCVD), are used to attempt growth of material with between 5 and 90 percent nitrogen, mixed polycrystalline layers with both zincblende and wurtzite crystals result. This immiscible region for the arsenide-nitride material of the present invention necessitates the use of growth techniques further away from thermodynamic equilibrium, such as atomic layer epitaxy, to obtain defect-free monocrystalline material with the required composition.
It has been found that lowering the growth rate of the AlGaAsN semiconductor material allows a greater concentration of N to be incorporated in the crystal. It is possible to increase the availability of N for incorporation by epitaxial growth using low-pressure metal organic vapor deposition (LP-MOCVD) utilizing N sources including hydrazine, phenol-hydrazine and metal-organic N sources. Ammonia (NH3) may be employed as an N source with catalysts such as arsine and phosphine used to increase the cracking efficiency of NH3. Alternatively, the N source can be pre-cracked. For example, in molecular beam epitaxy (MBE) an electron cyclotron resonator can be used to crack the NH3 molecule, resulting in a radical N. Plasma deposition systems can also be used to increase the cracking efficiencies of NH3.
It is possible to obtain the correct concentration of N relative to As by atomic layer epitaxy, a sequenced layering technique in which the growth monolayers containing N is interspersed with the growth of other monolayers containing As. The monolayers are thin enough to avoid defects due to mismatched lattices, as differing lattice constants between the monolayers tend to compensate for each other to produce a desired lattice match. For instance, five monolayers of GaAs can be grown for every monolayer of GaN, resulting in an overall Group V lattice site N concentration of approximately 17% and As concentration of about 83%. It is similarly possible to provide the relative concentrations of Al compared to Ga with such layering. Thus, for example, having two monolayers of Ga interspersed with four layers of Al results in a Ga concentration of approximately 33% and an Al concentration of approximately 67%. This process can be continued to form, for example, alternating quarter wavelength (115 nm) optically reflective layers or quantum wells formed of layers on the order of the electron wavelengths (5 nm) and other heterostructure lasing devices for which the exact depth of the layers is important.
The previous discussion has centered upon the growth techniques for alloys with N as the minority element on the Group V sublattice. Similar growth technology may be employed to grow alloys where N is the majority element on the Group V sublattice. However, the growth conditions themselves must be changed. Experiments have shown that the solubility of As in GaN is significantly greater than the solubility of N in GaAs; transmission electron microscopy has shown greater than 10% incorporation of As in GaN grown by conventional LP-MOCVD. Thus, for alloys with As content equal to or less than 10% MOCVD, either atmospheric or low-pressure, growth can be used with the ratio between N and As in the gas phase adjusted to provide the required As content. For alloy compositions greater than 10% As, multiple layer growth, as previously outlined, will be employed. For example, the layer structure required for the case of 10% As would employ nine layers of GaN, followed by a single layer of GaAs, etc.
Such materials can be used in a variety of applications. Semiconductor lasers and diodes can be created that emit light of frequencies that more than span the visible spectra, from infrared to ultraviolet. Conversely, photodetectors and solar detectors can be fashioned that utilize the wide range of available bandgaps to collect light of all colors. High temperature transistors, diode rectifiers and other electronic devices can also be produced using the high direct bandgap materials lattice matched to Si, SiC or GaP.
Thus, active layers of the direct bandgap material GaAsN, when used with cladding layers of GaN, AlGaN or AlGaAsN in a diode heterostructure, are prime candidates for optical emission which can be placed under biaxial compressive strain, a visible analog to the InGaAs strained layer technology currently employed in the AlGaAs material system for diode lasers or LED""s. At a border between two of the semiconductor materials, a p-n junction is formed by doping the respective materials with p-type and n-type dopants. Introducing dopants, both acceptors, such as C, Mg, and Zn, and donors, such as Si, Se, or Ge, into the GaAsN can be used to fabricate p-type and n-type doping, respectively. A cubic crystal lattice provides for easy formation of cleaved mirror and waveguide structures that feedback the emitted light to cause laser amplification. The resulting devices can efficiently emit coherent light spanning the visible spectra, including the blue region.